A variety of systems has been proposed and put into effect regarding the aligning of a reticle and a substrate such as a wafer in an exposure apparatus.
In the case of a TTR (Through The Reticle) system, a reticle is irradiated from above with an illuminating light source (e.g., laser light) for alignment, reflected light from a reticle fiducial mark (referred to as a “reticle mark” below) on a reticle or reticle stage and a stage fiducial mark (referred to as a “stage mark” below) on a wafer or wafer stage is detected by an optoelectronic detector, and the centers of the mark patterns, for example, are found from the images, thereby detecting a relative positional deviation between the wafer stage and reticle stage.
Reference will be had to FIG. 1 to describe this prior art arrangement in simple terms. Light from an optical source in an illuminating system 14 illuminates a mark 16, which is of the kind shown in FIG. 2A, on a reticle 2 through a half-mirror 17, a mirror 8 and the reticle 2. Since the mark 16 on the reticle 2 is a reflecting surface, the image of the mark 16 of reticle 2 arrives at a CCD camera 9 via the mirror 8 and half-mirror 17, as a result of which the image is optoelectronically converted. Meanwhile, the illuminating light that has passed through the reticle 2 illuminates a stage mark 15, which is of the kind shown in FIG. 2B, on a stage 4 via a projecting lens 3. Reflected light from the stage mark 15 arrives at the camera 9 again through the projecting lens 3, reticle 2, mirror 8 and half-mirror 17. Thus, a mark, which is a combination of the mark 16 on the reticle and the mark 15 on the wafer 5, is imaged by the camera, as shown in FIG. 2C.
In order to raise the density of integration of modem semiconductors, it has become necessary to expose the wafer to an extremely fine pattern of less than 0.15 μm. For this reason, the exposing light source used heretofore is a KrF laser, an ArF laser or an F2 laser. All of these lasers are of the pulse-output type.
Since light having a wavelength identical with that of the exposing light must be used in TTR measurement as well, a pulsed laser of a short wavelength has come to be employed.
If laser light is used as the illuminating light source for image sensing in order to detect relative positional deviation between the wafer and reticle stages, spatial coherence is high and a speckle pattern or interference fringes will occur when the wafer is irradiated with this light as is. Accordingly, the beam is oscillated or the phase of the speckle pattern or interference fringes is varied pulse by pulse by a rotary diffusion plate to irradiate the wafer with pulses a plurality of times, whereby the influence of the speckle pattern or interference fringes is eliminated by an integration effect. In a case wherein a wafer is exposed, several score or several hundred pulses are required to eliminate interference fringes or a speckle pattern. Methods of image sensing are described in detail in the specifications of U.S. Pat. Nos. 5,141,321 and 5,347,118.
As mentioned above, the illuminating light source of a semiconductor exposure apparatus uses a short wavelength to deal with the finer patterns used in modern semiconductor processes. This means that the effects of light absorption by the lens can no longer be ignored. More specifically, since the illuminating light and mark reflection light pass through the lens in both directions with regard to the stage mark 15, as shown in FIG. 1, the reflected light from the stage mark 15 is darker in comparison with the reflected light from the reticle mark 16. Since the reflected light from the reticle mark 16 does not make a round trip through the projecting lens 3, it is not affected by any absorption ascribable to the lens 3. If the reticle mark 16 and stage mark 15 are observed simultaneously, therefore, a difference in quantity of light will occur. If the reflected light from the stage mark 15 darkens to such an extent that an adequate amount of light is no longer obtained, signal contrast possessed by the obtained image may be insufficient and measurement precision may decline. If it is arranged to brighten the reflected light from the stage mark 15 in order to avoid this, then the reflected light from the reticle mark 16 will be too bright and signal saturation will occur.